It is known to detect excess temperature or fire using a bimetallic thermostat placed in a zone of an aircraft that is to be monitored, for example in its propulsion engine or in a transmission gearbox of a rotary wing aircraft. A resistor may be provided across the contact terminals that are opened or closed by the bimetallic strip.
In order to monitor a plurality of distinct zones, it is possible to connect in series a plurality of thermostats having contacts that are normally closed (NC); it is also possible to connect in parallel a plurality of thermostats having contacts that are normally open (NO).
It is also known to use detectors with changeover contacts combining a contact that is normally closed and a contact that is normally open.
British patent No. 1 343 819 describes a multi-zone fire detection loop comprising a plurality of detection units and a terminal resistor; each detection unit has a series fault NC contact and a short-circuit fault NO contact; in the event of a fire, closure of an NO contact short-circuits the terminal resistor, reducing the impedance of the loop and activating a loop relay; this activation is held by a holding contact controlled by the relay, so long as a switch for turning off the power supply to the loop is not actuated.
The loop also has a transistor acting in the absence of an alarm to pass sufficient current in the loop to power a second relay indicative of good operation; if the loop is opened, deactivation of the second relay serves to warn of the fault.
In order to avoid a fault detection unit “hiding” an alarm detected by another detection unit placed further “downstream” in the loop, a monitoring circuit is also associated with each detection unit; that circuit includes a lamp identifying which detection unit is at fault and serves to keep current flowing in the loop.
Such a detection loop is relatively complex and ill-adapted to monitoring multiple zones on board an aircraft.
Prior art fire detectors, and prior art systems incorporating such detectors, present too great a rate of false alarms, in particular from the NC contact detectors (normally closed).
The reliability of NO detectors is generally less than that of NC detectors, due to corrosion of the contacts of the bimetallic strip.
In order to protect the contacts of such expansion detectors, the bimetallic strip and its contacts can be housed in a sealed housing or compartment; this makes it more complicated to verify proper operation of the detector. When such verification is performed by heating the detector, that can result in changing the “changeover” temperature of the bimetallic contact.